This invention relates to a process for deashing coal and, more particularly, to a coal deashing process comprising producing agglomerated coal from crushed coal and a binder and at the same time removing inorganic minerals (hereinafter referred to as ash) in the crushed coal, wherein the binder is used in a reduced amount, the agglomerated coal is separated by means of a screen and recovered, and fine agglomerated coal contained in an ash slurry as an undersize is recovered by flotation.
A conventional oil agglomeration process (hereinafter referred to as OA process) is a process for removing ash from steam coal (hereinafter referred to simply as coal) and recovering coal.
This OA process comprises crushing coal so that 70 to 80% of the particles can pass through a 200-mesh screen, adding about 20 wt.%, based on pure coal, of a binder such as petroleum hydrocarbon oil and water to the obtained crushed coal to form an aqueous slurry of the crushed coal, agitating the slurry to allow the crushed coal particles to tumble and agglomerate into agglomerated coal and at the same time to allow ash particles in the crushed coal to disperse into the water.
However, this OA process has an economical disadvantage because the binder is used in a large amount.
Therefore, in order to reduce the use amount of a binder, a process in which the particle size of crushed coal was reduced to 6 mm or smaller was practiced. According to this process, the amount of a binder added can be reduced because the specific surface area of the crushed coal is increased.
However, the binder must be added in an amount of 8 to 20 wt.% also in this process, so that it has the following drawbacks:
A: When the agglomerated coal is fed as a fuel into a boiler fired with pulverized coal, it is necessary to pulverize the agglomerated coal to obtain a particle size distribution, for example, such that 70 to 80% of the pulverized coal can pass through a 200-mesh screen.
However, since a large amount of a binder is still contained in the agglomerated coal as mentioned above, the coefficient of friction of the agglomerated coal is decreased, which makes the pulverization of the coal difficult, so that the cost of power required for the pulverization is significantly increased.
FIG. 1 shows the relationship between the binder content (wt.%) and the power requirement (kWh/ton), wherein the curves A and B refer to two kinds of coal.
FIG. 1 clearly shows that the power requirement increases as the binder content is increased.
B: The pulverized coal obtained by pulverizing the agglomerated coal is piped to a boiler. However, since the agglomerated coal contains a large amount of a binder, the pulverized coal adheres to the inside wall of a transportation pipe, so that it can not be fed quantitatively to the boiler.
As a result, the combustion in the boiler becomes unstable, the amount of fine pulverized coal unburnt increases, and the combustion efficiency decreases. This makes it impossible to stably operate the boiler.
Table 1 shows the relationship between the binder content in the agglomerated coal and the amount of the pulverized coal adhering to the inside wall of a transportation pipe for the pulverized coal. FIG. 2 shows the relationship between the binder content in the agglomerated coal and the oxygen concentration in combustion exhaust gas.
Table 1 clearly shows that, when the binder content in the agglomerated coal is increased, the amount of the pulverized coal adhering to the inside wall of the transportation pipe is markedly increased.
In FIG. 2, the curve C shows the oxygen concentration in combustion exhaust gas produced when the pulverized coal is in a state of stable combustion and the curve D shows a case where the pulverized coal is in a state of unstable combustion due to the adhesion of pulverized coal to the inside wall of the transportation pipe.
It is clear that the curve D is markedly fluctuating as compared with the curve C.
TABLE 1 ______________________________________ Binder content in Amount of pulverized coal adhering agglomerated coal to the inside wall of a transporta- (wt. %) tion pipe (g/m.sup.2) ______________________________________ 6 1.9 to 7.5 8 52.5 ______________________________________
C: Because the agglomerated coal still contains a large amount of a binder, it forms lumps by compression when piled up in a coal yard, a silo or a hold. This makes its handling in subsequent steps difficult.
D: When an aqueous slurry of the crushed coal is agitated in the presence of a large amount of a binder, the agglomerated coal produced has a uniform diameter or a large particle size. When the agglomerated coal is piped, the critical velocity in a pipe is so large that the deposition of the agglomerated coal occurs in the pipe.
In order to prevent this deposition, it is necessary to increase the flow velocity of the agglomerated coal through the pipe, which leads to an increased power for transportation.
E: The ratio of the cost of a binder used to the total manufacturing cost of the agglomerated coal amounts to as high as 30 to 40%, so that the OA process is very questionable from the viewpoint of economy.
F: The particle size of the agglomerated particles is smaller because of a smaller amount of a binder as compared with that in the OA process, so that the percentage recovery of agglomerated coal is as low as about 70% when recovered by screening.